Fix er 3 and 7 buids in latest erbuim (subscribers counter has changed type)

er-13 has changed the type of notify_subscribers counter to int32_t from uint16_t which breaks builds of WITH_COAP=3 or 7.

This patch changes the 03 and 07 engine to int32_t.

An alternate fix would be to switch out the type in erbuim.h based on
WITH_COAP.

Or, maybe better, would be to drop old COAP versions.

@mkovatsc thoughts?